A main steam valve in a steam turbine is used under harsh conditions of high temperature and high pressure, and further takes a task of controlling high-speed steam flow. Under high temperature, the surface of a metal is in an activated state to react with the high-temperature steam present in the atmosphere to produce an oxide film. The oxide film thus produced has different adhesion strength to the base metal, depending on the composition of the base metal and the atmospheric conditions, and undergoes delamination at every repeated opening-closing movement of the valve. The delaminated matter is deposited locally in the recessed portions of the valve shaft by the sliding of the valve shaft so as to fill in the clearance between the bearing and the valve shaft. Consequently, the sticking of the valve shaft is caused as the case may be. Accordingly, at the time of the periodic inspection of a steam turbine, it is necessary to conduct maintenance work for removing the oxide film by disassembling the members surrounding the valve shaft. Additionally, the generation amount of the deposition substance is anticipated, and accordingly the clearance between the valve shaft and the bearing is set at a sufficiently large value. Therefore, the steam amount leaking from the portion surrounding the valve shaft is increased to cause problems such as the decrease of the thermal efficiency of a whole plant.
As a method for solving the above-described problem, a surface-hardened layer based on nitriding has hitherto been provided on the outer surface of the valve shaft; however, the nitride layer is associated with a drawback that the nitride layer has characteristics of being decomposed and softened at about 500° C. or higher, and additionally, the thickness of the nitride layer is extremely thin, and hence the dissipation of the nitride layer leads to rapid development of abrasion.
As described above, even when an oxide film is produced, the valve shaft and the bearing are required to hold the clearance therebetween to such an extent that no disturbance is imposed on the operation of the valve. Additionally, such a valve shaft and such a bearing have been formed of a material such as a low-alloy steel (a Cr-Mo-V steel), 12 Cr stainless steel and an austenite stainless steel, and subjected to a surface-hardening treatment based on nitriding for the purpose of preventing abrasion, sticking and the like. The clearance between the valve shaft and the bearing provided to prevent sticking is set to be large with a surplus space, and hence there occurs a danger that the valve shaft is vibrated by the steam flow, the end of the bearing is abraded in a bell mouth shape and the vibration of the valve shaft is thereby further increased.
For the above-described problems, Patent Document 1 has proposed a valve gear in which building-up welding with a cobalt-based alloy is applied. In this valve gear, at least first two layers of the built-up layers are formed of a Co-based alloy (Stellite No. 25) composed of, in terms of weight, 9 to 11% of Ni, 19 to 21% of Cr, 14 to 16% of W, 0.05 to 0.15% of C, a small amount of Fe and impurities and the balance composed of Co, and an additional layer on top of these layers is formed of a Co-based alloy (Stellite No. 6) composed of, in terms of weight, 3 to 6% of Ni, 26 to 32% of Cr, 0.9 to 1.4% of C, 3 to 6% of W. a small amount of Fe and impurities and the balance composed of Co or a Co-based alloy (Stellite No. 21) composed of, in terms of weight, 4% or less of Ni, 25 to 29% of Cr, 0.2 to 0.3% of C, 5 to 6% of Mo, a small amount of Fe and impurities and the balance composed of Co. It is to be noted that Stellite is a registered trade name of Deloro Stellite Company
Patent Document 1 has presented an example of the use of TIG welding as building-up welding, and on the other hand, Patent Document 2 has proposed a formation of a welded layer of a Co-based alloy on the outer surface of the valve shaft in contact with the bearing by means of plasma powder building-up welding.
Patent Document 3 has also proposed: a formation of a cladding hardened layer on the contacting-to-bearing surface of a valve shaft constituting a driving unit of a steam valve, by continuously feeding a cobalt-based hard alloy powder, and irradiating the powder with a laser beam to successively melt the powder to form a cladding hardened layer; and a formation of a cladding hardened layer on the contacting-to-bearing surface of a valve shaft constituting a driving unit of a steam valve, by thermally spraying or applying a cobalt-based hard alloy powder onto the contacting-to-bearing surface to form a hard alloy layer, and by thereafter irradiating the thus formed hard alloy layer with a laser beam to melt the hard alloy layer again to form the cladding hardened layer.
Patent Document 1 and Patent Document 2 both disclose the reduction of the oxide film generation by use of Co-based alloys, typically Stellites.
Patent Document 1: Japanese Patent Laid-Open No. 59-169696
Patent Document 2: Japanese Patent Laid-Open No. 60-17206
Patent Document 3: Japanese Patent Laid-Open No. 6-174126